Abstract: The micro-cracks and porosity of concrete structures are very common problems due to the fact that this material has a high permeability which allows water and other aggressive media to enter thus leading to deterioration. The use of traditional organic polymer based crack sealers is a common way of contributing to concrete durability. However, the most common organic polymers have some degree of toxicity and are not environmental friendly. Shortcomings of conventional surface treatments have drawn the attention to alternative techniques for the improvement of the durability of concrete. Recently research done at JNTU Hyderabad has shown that specific species of bacteria can actually be useful as a tool to repair cracks in already existing concrete structures. Calcite precipitation due to microbial chemical process by specific alkali resistant microorganisms can act as a self healing agent when induced into concrete. This mechanism is of great interest for repair in concrete structures without human intervention. A new type of alkaliphilic aerobic microorganism belonging to Bacillus species, which when added to concrete enhances the strength and durability characteristics of concrete structures significantly due to growth of filler material called calcite(CaCO3) within the pores of the cement–sand matrix leading to pore refinement and enhanced concrete microstructure. This paper reports the effects of bacterial carbonate precipitation (bio-deposition) on the strength and durability of concrete specimens of ordinary (M20), standard (M40) and High strength (M60) grades.. Keywords: Bacterial Concrete, Bacillus Subtilus, Bio-mineralization, self healing concrete, SEM.

INTRODUCTION Reinforcement corrosion is one of the major durability problems, mainly when the rebar in the concrete is exposed to the chlorides either contributed from the concrete ingredients or penetrated from the surrounding chloride-bearing environment. From the perspective of durability the cracks formed should be repaired conventionally using epoxy injection, latex treatment etc or by providing extra reinforcement in the structure to ensure that the crack width stays within a certain limit. Especially with current steel prices on steep rise, providing extra steel is not economically viable. Use of synthetic agents such as epoxies for remediation of cracks in these structures introduces a different material system of doubtful long term performance and moreover they may damage the aesthetic appearance of the structures. Sometimes repair is carried out in the areas where it is not possible to shut down the plant or hazardous for human beings such as nuclear power plants where fuel storages should be leak proof , repair of waste water sewage pipes etc. Hence, in treating surfaces of structures with strategic and historic heritage importance, self healing materials could be an ideal choice. So, If in some way a reliable method could be developed that repairs cracks and enhances corrosion resistance in concrete automatically (self healing), which could increase and ensure durability and functionality of structures enormously results in the conception of “Bacterial Concrete”[1]. An innovation based on biomimicry and biotechnology has lead to the method of sealing up of micro cracks in concrete by itself using microorganisms as a sustainable alternative to other available chemical methods of crack repair such as epoxy treatment etc [2]. Compared with the commonly used repair method which follows the procedure of detection, monitoring and repair, the self-healing method is cheaper over the structure’s life -cycle since the later maintenance would be greatly saved. Organic polymer, super absorbent polymer, expansive agents and so on are being investigated as self-healing materials for cracks. Another alternative self-healing material is microbial carbonation precipitation [3]. Some bacteria can produce or induce bio-minerals during their growth and metabolism [4]. Under suitable conditions, most bacteria are capable of inducing carbonate precipitation. The precipitated bio-CaCO3 has a good potential to be used to heal concrete cracks because it is natural, environmentally friendly and compatible with the concrete matrix [5]. Compared to natural carbonation of concrete, bio-deposition is a relatively quick process. Natural carbonation occurs from the dissolution of atmospheric CO2 in the pore solution and formation of CaCO 3 from CSH or portlandite. In the biodeposition treatment however, calcium ions are also provided by an externally added calcium source, while the carbonate ions result from the microbiological hydrolysis of amino acids [6]. MECHANISM OF BIO-BASED CONCRETE CRACK REPAIR In nature, microorganisms can induce calcite mineral precipitation through nitrogen cycle either by ammonification of amino acids/ nitrate reduction/ hydrolysis of urea [7]. Bacillus subtilis JC3 is able to precipitate calcium carbonate (CaCO3) in its micro-environment by the ammonification of amino acids into ammonium (NH4+) and carbonate (CO32-) ions. The precipitated bio-CaCO3 has a great potential ability to heal concrete cracks because it is natural, environmentally friendly and compatible with the concrete matrix [8]. Bacillus subtilis JC3 a non-pathogenic alkalophilic microorganism commonly found in soil and is known to deposit the calcite minerals when it is supplied with nutrients and 1

Table 1: Quality of concrete based on Ultrasonic pulse velocity Velocity Quality of concrete > 4. Table2 shows Guidelines for qualitative interpretation of rebound hammer test results as tabulated in Table 6.5 km/s medium < 3. moisture and carbon dioxide at the crack face will activate the microorganisms and will provide the conditions favorable for growth [9].5 km/s excellent 3. Harder the surface of the material tested. The microorganisms will deposit calcium carbonate.5 km/s Good 3.subtilis Cell.e.0 km/s doubtful Rebound Hammer As per IS 13311. 10 5/ml cell concentration were used) with media as mixing water. Effect of Bacterial Cell Concentration on Strength Effect of cell concentration of Bacillus subtilis JC3 on the strength is studied by determining the compressive strength of standard cement mortar cubes incorporated with various bacterial cell concentrations as per IS: 4031-part 6 as shown in Figure 1.Ca2+ + CO32. ensuring the continual effectiveness of the microorganisms in filling up cracks at the same location [10]. voids & other imperfections and changes in concrete structure with time. velocity is co-related to strength and quality of bacterial concrete specimens as shown in Table 5. Ultrasonic Pulse Velocity Test (USPV) The test is performed as per IS code 13311 (Part 1) 1992 to find out the homogeneity of bacterial concrete. greater is the rebound. Strength Studies on Bacterial Concrete To study compressive strength characteristics. The combination of the pH drop and a flow of oxygen. Modulus of Elasticity is computed from the stress.subtilis Cell → B. The compressive strength of the bacterial concrete cubes at 28 days is compared with corresponding controlled specimens. standard cubes (100mm x 100mm x 100mm) were cast with distilled water and the require amount of microorganisms (i. and as the crack fill up.subtilis Cell.subtilis Cell. Once a crack forms. this test measures the surface hardness of concrete and is co-related to the strength and quality of concrete. which helps to understand the crack healing ability of Bacterial concrete and its characteristics (Strength and Durability).The chemical equations involved in microbial activity are: Ca2+ + B.→ B. Table 2: Quality of concrete based on Average Rebound Hammer Average rebound number Quality of concrete > 40 Very good hard layer 30 to 40 Good layer 20 to 30 Fair < 20 Poor concrete
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.National Conference on Advances in Civil Engineering and Infrastructure development (ACEID-2014) 6-7 Febrauary 2014.5 to 4.0 to 3.Ca2+ CH3CH(NH2)COOH (Peptone) + ½O2 ---------> C2H2 + H2CO3 + NH3 H2CO3 ----------> H+ + HCO3NH3 + H2O --------> NH4+ + OHB. In this method. Hyderabad
right conditions to grow. causing the microorganisms to hibernate again. Vasavi College of Engineering. presence of cracks. The ammonia liberated will provide the conditions favorable for growth and also maintains the pH of concrete.CaCO3 EXPERIMENTAL INVESTIGATIONS The main aim of the present experimental investigations is to obtain specific experimental data. Similarly 28 days split tensile strength and flexural strength is determined from cylinders (150 mm x150 mm x 300 mm) and prisms (100mm x 100mm x 500mm) respectively. The bacteria introduced into the concrete during mixing process will form spores in the highly alkaline environment of concrete. Bio-mineralization by Ammonification (Ammo acid degradation) is mediated by Bacillus subtilis JC3. the pH level at the cracked surface will drop due to the exposure to air. the supply of oxygen and carbon dioxide will be interrupted. (Part 2): 1992. Ammonification usually occurs under aerobic conditions (known as oxidative deamination) with the liberation of ammonia (NH 3) or ammonium ions (NH4) when dissolved in water.strain curves of controlled and bacterial concrete.

In upstream reservoir is a 3. The following formula. In the AASHTO T277 (ASTM C1202) test (Electrical indication of concrete’s ability to resist chloride ion penetration ). are used to classify the concrete in terms of their permeability as per the recommendations of the Concrete Society. The extent of deterioration at each corner of the struck face and the opposite face is measured in terms of the acid diagonals (in mm) for each of two cubes and the “Acid Attack Factor” (AAF) per face is calculated as follows: AAF = (Loss in mm on eight corners of each of 2 cubes) / 4 Acid Durability Factors (ADF). in Table 8.3 M NaOH solution (chloride free) (Anode). weight.2.
3
. indicating the degree of resistance of the specimen to chloride ion penetration as shown in Table 7. Average permeability concrete: (1 to 5) x 10 -12 m-/s. Q = 900(I0+2I30+2I60+2I90+2I120+…+2I300+2I330+I360) Where.4. the durability factors are proposed by the author. The response of the specimens to the solutions was evaluated through change in appearance.0% NaCl solution of 2. The total charge passed is determined and this is used to rate the quality of the concrete according to the criteria rating mentioned in the code. In this project. The total charge passing through from one reservoir to another reservoir through centrally placed concrete specimen in 6 hrs was measured.000 . I0 = Current reading in amperes immediately after voltage is applied.000 Very Low 100 . one litre of nutrients mixed bacterial culture costs Rs 60.000 Moderate 2. a water-saturated. Acid Attack Factors (AAF). obtained from Rapid chloride ion penetrability test was used to calculate Chloride Migration Diffusion Coefficient in steady state conditions fr om Berke’s empirical Equation.000 Negligible < 100 Acid Attack Resistance To study durability characteristics. compressive strength. Low permeability concrete :< 1 x 10-12 m2/s. 50-mm thick. with the philosophy of ASTM C 666–1997. at an interval of 30 min. The mode of transport of chloride ion through concentration gradient is called Diffusion. Chloride ion penetration is one of the main parameter affecting the durability of reinforced cement concrete structures. DC=0. In the present investigation.4N concentration (Cathode) and in the downstream reservoir is a 0. Sr = relative strength at corresponding N days. Cost Analysis The cost/benefit analysis of bacterial concrete balances the increased cost of the concrete against substantial repair material costs. enhanced durability and aesthetic benefits. Chloride diffusivity in terms of charge passed of bacterial concrete using Rapid Chloride Penetration Test (RCPT) as per ASTM C 1202 is investigated.84 m2/s The calculated diffusion coefficient values. at the start of the test). as the basis.000 Low 1. The benefits are apparent at strength and performance of the finished product. Only expensive component in the development of bacterial concrete is nutrients. Table 3: RCPT and Resistivity Criteria Ratings Permeability Class Rapid Chloride Permeability Charge Passed (Coulombs) as per ASTM C1202 High > 4. apart from permeability.e.0103 x 10-12 x Q0. the author derived the “Acid Durability Factors” directly in terms of relative strengths. based on the trapezoidal rule can be used to calculate the average current flowing through one cell.1. Vasavi College of Engineering. The most important concrete characteristic.000 . Q = current flowing through one cell (coulombs) . The rate at which chloride ions penetrate into concrete determines the time period after which the passivity of reinforcing bars begin to break down. N = number of days at which the durability factor is needed and M = number of days at which the exposure is to be terminated. the specimens are subjected to 3% and 5% concentrated solutions of HCL and H 2SO4 using acid immersion test. thickness and solid diagonals. is diffusion. Q in coulombs. UK: High permeability concrete: >5x10-12 m2/s. The “Acid Durability Factors” (ADF) can be designed as follows: Acid Durability Factor (ADF) = Sr (N / M ) where. and It = Current reading in amperes at t minutes (30 min interval) after voltage is applied The electric charge passed. Hyderabad
Chloride Diffusivity Studies The chloride resistance of concrete is governed primarily by the pore structure and the concrete diffusivity. The relative strengths are always with respect to the 28 days value (i.National Conference on Advances in Civil Engineering and Infrastructure development (ACEID-2014) 6-7 Febrauary 2014. For determining the resistance of concrete specimens to aggressive environment such as acid attack. 100-mm diameter concrete specimen is subjected to a 60 V applied DC voltage for 6 hours. percentage weight loss and strength loss at various days of immersion are evaluated.

28 0.97 0.48 56.13 0.31 0.Split Tensile and Flexural strength of concrete at 28 days increased by about 14-22% and 21-30% respectively.27 65. In bacterial concrete.52 0.13
0.91
56.25 62. So. Diffusion Coefficient (DC) of chloride ions decreases with increase in higher grades in normal concrete but with introduction of bacteria into concrete further decreased the effective diffusion coefficient.50 98.61 28.05 Bacterial Concrete 33. Vasavi College of Engineering.39
0.28 0.86 0.13 0.34 0.71
0.72 1.35 92.53 0.96 0.07 0.66 91. The relationship between stress and strain is important in understanding the basic elastic behavior of concrete in hardened state.33 0.11 98.01 0.59 0.50 33. Grades of Normal concrete have higher current flow when compared to Grades of Bacterial concrete.09 0. the rebound hammer values are more because of greater elastic rebound. as it gains strength.66 28.47 0.01
0.81 0.99 65.50 0.07 0.16 0. Durability studies carried out in this investigation through acid attack resistance for all grades of concrete with 3% and 5% concentrated solutions of H2SO4 and HCL revealed that Bacterial Concrete is more durable in terms of “Acid Durability Factors” and less attacked in terms of “Acid Attack Factors” when compared to the controlled concrete.16 0.38 0.48
52.31 0.56 95.77 99. In order to assess particle continuity inside the concrete specimen.16 0.92
0.54 0.38 54.National Conference on Advances in Civil Engineering and Infrastructure development (ACEID-2014) 6-7 Febrauary 2014.53
Controlled Concrete 33.05 82. which fills up the pores in the concrete making dense microstructure. hardness increases and as a result.56 33.16 0.29 64.97 0.95 63.52 98.56 32.97
0.66
DISCUSSION OF RESULTS It is observed that the Compressive Strength of cement mortar showed significant increase by about 17% for cell concentration of 105 cells per ml of mixing water. With the addition of bacteria the Compressive Strength at 28 days showed significant increase by 16-30 % for all grades of concrete.91 65.19 0.28 65. Hyderabad
60 90 (=M) 30 60 90(=M)
62.84 99.66 62.
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.41 62. The impermeability of concrete can be represented by the rate of flow or diffusion coefficient of chloride ions through the unit area of concrete. The percentage of Compressive Strength is improved as the age of the concrete increases due to continues bacterial calcification of Bacillus Subtilus JC3.95
0. It is noted that pores are partially filled up by material growth with the addition of the bacteria.09 0.99 96.84
Bacterial Concrete
High Strength Grade (M60) Concrete Days of immersion N 3% H2SO4 ADF AAF 3% HCL ADF AAF 5% H2SO4 ADF AAF 5% HCL ADF AAF
30 60 90 (=M) 30 60 90(=M)
33. USPV test is recommended. for the further investigation bacteria with a optimum cell concentration of 105 cells per ml of mixing water is used. Bacterial concrete will have dense microstructure due to precipitation of mineral in pores of concrete.17 83.13 32.98 65.39 33.549
63.91 0.13 91.61 71.09 0.33 76.46 0.76 99. Bacterial Concrete mixes have shown improved stress values for the same strain levels compared to that of conventional concrete mixes. It is observed that Modulus of Elasticity(E) is relatively more for all grades of concrete in which bacteria is induced than the controlled concrete by about 35-65%.34 0.59
33.24 33.00 96. Reduction in chloride ion permeability values indicates that bacteria induced concrete has shown between 85% to 90 % higher resistance against the chloride ion movements in bacterial concrete as compared to the chloride movements in normal concrete.59 0.78 1. Reduction in pore due to such material growth will obviously increase the material strength.

S. Bacteria induced concrete has substantially high modulus of elasticity. So bacteria incorporated concrete has increased packing density and reduced capillary porosity. Day JL. Torino. Bang SS. De Belie. Ramesh KP. High Strength Concrete. Verstraete. Bang. S. The addition of Bacillus subtilis JC3 strain increases the compressive strength. 2000.” pp. 28(2001) 404-09 3. and Ramakrishnan V. SS. Ramakrishnan V. Williams A E. Bachmeier K. 597– 617.National Conference on Advances in Civil Engineering and Infrastructure development (ACEID-2014) 6-7 Febrauary 2014. “Bacterial calcification. Calcite mineral precipitation results in less capillary porosity in the hardened paste and hence a greater strength. Galinat JK. V. and Bang SS. Ramikrishnan V. Montreal. and W. In bacterial concrete significant reductions in water permeability and chloride ingress have been observed along with its increased resistance to attack by aggressive chemicals. it has been revealed that bacterial concrete has better resistance against strength deterioration for all curing conditions and curing ages. Ramakrishnan V. South Dokata School of Mines and Technology. Knorre. The strength of the paste will be limited by the flaws that form the weakest link. 3. “Remediation of concrete using microorganisms”. Warminton J and Bang. 2. In R. In order to improve the strength of the paste as a whole. W.S. Duke EF. Bacterial Concrete. M Awramik (eds. REFERENCES 1. In bacterial concrete. Furthermore the effect of mineral precipitation homogenously in bacterial concrete leads to a reduction in inhomogeneities within the paste and hence improved paste strength.From the investigation. S. Microbial Sediments.Ramakrishnan. 6. Springer-Verlag. Ramkrishnan. It is revealed that bacterial concrete is more durable in terms of “Acid Durability Factor” than conventional concrete and less attacked in terms of “Acid Attack Factor” than conventional concrete. 1998.. Canada. 8. From the durability studies. 5. the percentage weight losses and percentage strength losses revealed that Bacterial concrete has less weight and strength losses than the conventional concrete against any acid attacks. bacteria with a cell concentration of 10 5 cells per ml of mixing water was used in the investigation. Deposition of a layer of calcite on the surface of the specimens resulted in a decrease of capillary suction. V. 2005. Berlin.
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. Ramachandran. SEM Investigation of Microbial Calcite Precipitation in Cement Proceedings of the 22nd International Conference on Cement Microscopy. and Bang SS. Proceeding of the International Conference on High Performance. The incorporation of microorganisms into concrete confers enhanced durability on the concrete. 293-305. So. 2008. induction of microorganisms inside the concrete has enormous effect on the porosity within the cement matrix paste. 38: 1005 -1014. Krumbein. 93(2002) 171-181. K. “A novel technique for repairing cr acks in high performance concrete using bacteria”. Perth. 4234 pp. Ramakrishnan V. Panchalan RK. Microbiologically induced sealant for concrete crack remediation Proceedings of the 16th Engineering Mechanics conference. Deo. Hyderabad
CONCLUSIONS Based on the present experimental investigation. 2. 4. S. Therefore bacterial concrete is a new approach to enhance the strength and durability of the concrete economically. Smart Materials. WA. on the particle size distribution of the crystalline phases and on the presence of in-homogeneities within the hydrated paste due to mineral precipitation. 9. The calcite crystals formed will glue together the hydrated particles which reduce the interstitial porosity between them.K. Urease activity in Microbiologically-induced calcite precipitation” Journal of Biotechnology. Vol. H. 10. Australia. USA.S. Improvement of concrete durability by bacterial mineral precipitation” Proceedings ICF 11. E. Calcite precipitation induced by polyurethaneimmobilized Bacillus pasteurii” Enzyme and Microbial Technology. pp. 4. ACI Materials Journal 98 (1) (2001) 3–9. Seattle. Bang. 5. Proceedings of SPIE. Debrouwer. Riding and S. be the inhomogeneities or capillary pores. Bang. the following conclusions are drawn 1. pp. Germany. 7. D. 7.S. 2003. “Bacterial carbonate p recipitation improves the durability of cementitious materials. 25 -31.” Cement Concrete Res. De Muynck. 168-176. This reduced capillary porosity also favours the formation of finetextured hydration products with optimized particle size distribution of the cementitious materials in order to increase the potential packing density. Panchalan RK. S. Italy. Santhosh KR. The addition of Bacillus subtilis JC3 strain improves the hydrated structure of cement in concrete for a cell concentration of 105 cells per ml of mixing water. all such flaws must be minimized. 2000.). Vasavi College of Engineering. 6. split tensile strength and flexural strength of concrete when compared to controlled concrete. N. and W.